Earth's oceans contain one of the largest reservoirs of mobile carbon on the planet, i.e., >˜38,000 gigatons (Gt) of inorganic carbon, which mobile carbon constantly exchanges with atmospheric carbon. The amount of exchange varies with the area of ocean, with some areas outgassing carbon at about 100 Gt/yr. and other areas taking up more than about 100 Gt carbon/yr. On average, the oceans take up a few more Gt/yr. than they emit, and as a consequence of this natural imbalance, the oceans have presently absorbed about one third of all the carbon dioxide (CO2) which has been formed due to burning of fossil fuels.
While a portion of the carbon uptake by ocean waters results from purely physical-chemical mechanisms, i.e., a higher atmospheric CO2 concentration increases the gradient (difference) between the ocean surface CO2 concentration and the atmosphere thereby increasing net carbon uptake by the ocean, another portion of the carbon uptake is caused by the ocean biology. The latter carbon uptake mechanism is amenable to stimulation by human activity.
The amount of carbon present at the ocean surface is largely controlled by the balance between carbon which is “upwelled” to the surface from below and carbon which is removed from the surface by biological processes. One method or mechanism for altering the long-term balance between upwelled and removed (“sequestered”) carbon and increasing carbon uptake by the oceans comprises stimulating the creation of organic (i.e., carbon-containing) matter by supplying marine plant species (phytoplankton) with nutrients necessary for growth. Over most of the oceans, available nitrogen usually constitutes the most severely limiting plant nutrient, primarily because growth of the existing marine plants consumes nearly the entire amount of biologically available reactive nitrogen (i.e., nitrate, nitrite, ammonia, urea, and organic nitrogen). While addition of more reactive nitrogen to the oceans will effect additional marine plant growth resulting in increased CO2 uptake, it is not possible or practical to directly supply sufficient nitrogen-based fertilizer to the oceans due to the prohibitive cost and the enormous mass of fertilizer required.
An alternative mechanism for providing reactive nitrogen species necessary for growth of marine plants involves conversion of diatomic nitrogen gas (N2) into organic nitrogen by certain organisms, e.g., photosynthetic bacteria, via a natural fertilization process which occurs in the upper, i.e., sunlit, ocean layer(s). This mechanism can be stimulated by addition of trace amounts of iron (Fe) and other minerals and chemical compounds which normally are limiting with respect to growth of the organisms. In practice, this mechanism shifts the partitioning of carbon between the oceans and the atmosphere until the thus formed reactive nitrogen (usually stored as nitrate) is converted back to N2 gas via a complementary bacterial process termed “denitrification”. The timescale for nitrogen fixation resulting from this process ranges from several decades to millennia. In addition to this mechanism, nitrogen fixation in some major ocean areas is limited by the availability of phosphate.
Stimulation of nitrogen fixation by iron and other minerals, as described above, forms a pathway for influencing natural oceanic systems to induce a net shift of carbon from the atmosphere to the ocean. As indicated supra, the premise is that stimulation of nitrogen-fixing marine organisms by addition of trace substances (e.g., metals such as iron) that are either lacking or present in insufficient amount will both increase the total amount of nitrate present in the ocean and reduce the amount of residual phosphate present in the surface waters in the portions of the ocean that contain very low amounts of nitrate while containing measurable phosphate. The net effect of the stimulation process would be to shift a measurable amount of carbon away from the ocean surface into the deeper sea. As a consequence, the carbon shift from the surface to deeper regions of the ocean will influence the ocean/atmosphere partitioning of carbon and cause CO2 to either enter the oceans more quickly in the plant growth-stimulated locations or to leave (i.e., outgas from) the plant growth-stimulated locations, depending upon the natural direction of CO2 flux (flow) in that location.
It is expected that carbon sequestered by the above mechanism will be retained in subsurface (i.e., deeper) strata of the oceans due to continually occurring natural marine biological processes. Thus, every time new nitrate, with its newly sequestered carbon, is returned to the illuminated surface waters, the new nitrate and newly sequestered carbon are taken up by the marine plants and returned to the deeper strata. The process will continue until the nutrients are upwelled in an area of incomplete nutrient use (i.e., high nutrient, low chlorophyll or “HNLC” regions) where iron depletion occurs before nitrate and/or phosphate depletion, or until that water passes through an area of low oxygen content where denitrifying bacteria selectively remove the nitrate. In the latter instances, subsequent upwelling no longer stimulates the same plant biology and the CO2 is once again in solution at the ocean surface for an extended time period. The result is re-equilibration with the atmosphere, and as a consequence, the interval (longevity) of sequestration of extra carbon via stimulation of nitrogen fixing organisms is expected to depend upon the time scales of mixing and circulation, estimated to be in the range from about 20 years to millennia. A small fraction of the carbon is expected to remain sequestered in sedimentary deposits.
Generally, organic matter that leaves the vicinity of the ocean surface is consumed by organisms present in the underlying water column. Most organic matter is remineralized within a few hundred meters of the surface and is available for upwelling back to the surface within 1-10 years, whereas organic matter which sinks more deeply below the ocean surface remains there as inorganic carbon and nutrients for centuries to millennia, depending upon the depth and degree of ocean mixing. At a given time, the upwelled water contains a mix of recently remineralized nutrients and nutrients which remineralized centuries ago. If the average depth of remineralization increases, the upwelled water will for some interval thereafter have less nutrient available from recent pools and at a later interval have more nutrient as the deeper pools return to the surface. This phenomenon effectively results in storage of the carbon by placing more of it in older, slower-to-return water masses. The processes or factors which control remineralization interval scales are unknown, but they do vary naturally and may be amenable to manipulation. In an extreme case, if the organic matter can be caused to reach the ocean floor, it can become part of the geological pools, whereby carbon may be sequestered for very long intervals (e.g., millennia to millions of years).
Another mechanism for carbon sequestration involves increasing the carbon content (“carbon richness”) of sinking organic matter. In most of the world's oceans, the carbon-to-nitrogen-to phosphorus (C:N:P) ratios are similar. Thus, when organic nitrogen is converted to nitrate, most of the carbon associated therewith is converted to inorganic carbon. When the nitrate-laden water is returned to the illuminated surface layer of the ocean, the carbon requirements of the plants can be satisfied by the carbon in the water without necessity for additional carbon to be supplied from the atmosphere. However, some carbon compounds are more carbon-rich than others and the stimulation of high carbon-nitrogen or carbon-phosphate plants is expected to increase the amount of carbon removed from the water. If this carbon is remineralized along with the nutrients, it will again be present in water returned to the surface and will remain therein only if the high C:N organic matter is reformed. Therefore, if this carbon is not re-used but stored as organic carbon or even buried, it will be effectively removed (sequestered) from the atmosphere for long time intervals. While the mechanisms that control the C:N and C:P ratios of sinking materials are unclear, variation thereof does occur naturally.
Inasmuch as the storage of anthropogenic carbon in ocean waters is currently of great interest as a potentially economically viable approach for addressing and mitigating the increasing problem of global warming attributed to the increasing content of greenhouse gases such as CO2 in the earth's atmosphere, and in view of the foregoing, there exists a great need for improved methodology for quantitatively assessing and verifying the amount and predicted duration of carbon storage/sequestration afforded by stimulation of blooms of nitrogen fixing organisms. Accordingly, the present disclosure provides a comprehensive methodology for quantitative determination of the efficacy and predicted longevity of carbon sequestration in ocean waters afforded by nitrogen fixation.